2012/10/21 Tejun Heo <tj@xxxxxxxxxx>:
> Hello, Frederic.
>
> On Sat, Oct 20, 2012 at 02:21:43PM -0400, Frederic Weisbecker wrote:
>> CPU 0
>> CPU 1
>>
>> cgroup_task_migrate {
>> task_lock(p)
>> rcu_assign_pointer(tsk->cgroups, newcg);
>> task_unlock(tsk);
>>
>> write_lock(&css_set_lock);
>> if (!list_empty(&tsk->cg_list))
>> list_move(&tsk->cg_list, &newcg->tasks);
>> write_unlock(&css_set_lock);
>>
>> write_lock(&css_set_lock);
>> put_css_set(oldcg);
>> list_add(&child->cg_list, &child->cgroups->tasks); (1)
>
> Man, that's confusing. :)
Sorry and I'm currently stuck in some airport and too lazy to reorder
the above lines :)
>
>> On (1), child->cgroups should have the value of newcg and not oldcg
>> due to the memory ordering implied by the locking of css_set_lock. Now
>> I can't guarantee that because I'm no memory ordering expert. And even
>> if it's safe, it's so very non obvious that I now agree with you:
>> let's revert the patch and restart with a better base by gathering
>> all the cgroup fork code in the current cgroup_post_fork place.
>
> Aye aye, let's move everything to cgroup_post_fork() and then we don't
> have to worry about grabbing task_lock multiple times.
Agreed. and Acked-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
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